1. Field of the Invention
The present invention is directed to a treatment system for the remediation of polychlorinated biphenyls (PCBs), chlorinated pesticides, and other halogenated compounds. In one embodiment, the treatment system comprises a plurality of zero-valent metal particles and a hydrogen donating solvent.
2. Description of Related Art
PCBs are a group of synthetic aromatic compounds with the general formula C12H10-xClx. PCBs are among the most persistent, bioaccumulative, and toxic compounds and are responsible for the primary risk at numerous sediment sites. PCBs are a group of synthetic aromatic compounds that were historically used in industrial paints, caulking material, and adhesives, as their properties enhanced structural integrity, reduced flammability, and boosted antifungal properties. PCBs have been used in many industrial applications because of their robust physical and chemical properties such as their resistance to acids, bases, and oxidation, their excellent dielectric characteristics, and their thermal stability at high temperatures (up to 350° C.). When PCBs were released into the environment, they were sorbed to particulate matter that was then dispersed over large areas. PCBs can be introduced into the food chain by the uptake of contaminated soils by biota and humans can directly inhale or absorb PCBs by dermal contact.
Although the United States Environmental Protection Agency (USEPA) has banned the manufacture of PCBs since 1979, PCBs are still present in the environment posing possible adverse health affects to both humans and animals. Prior to the USEPA's ban on PCB production, PCBs were commonly used as additives in paints and asphalt-based adhesives that were subsequently applied to a variety of structures. Governmental facilities constructed as early as 1930 utilize PCB-containing binders or PCB-containing paints, which are now leaching into the environment and posing ecological and worker health concerns. PCBs have been found in at least 500 of the 1,598 National Priorities List (Superfund) sites identified by USEPA. Many of the most costly cleanups are at sediment sites dominated by PCB contamination. Additionally, PCBs can still be found in the paints located on NASA property at a number of NASA Centers. The PCB and metal levels in painted structures on Kennedy Space Center have been documented to be as high as 31,000 ppm. PCBs have been introduced into the NASA work environment via improper disposal and accidental leaks from transformers, heat exchanges, and hydraulic systems. Numerous NASA Centers have older metal structures upon which paints containing PCBs were applied. These painted structures are posing worker and ecological health hazards and, in several instances, are now considered a TSCA-level (Toxic Substance Control Act) waste. Some of the impacted structures could be refurbished and utilized for new programs, but because the paint currently on the structures is heavily laden with PCBs, the programs are unable to reuse or even discard these structures without significant cost.
The removal of contaminants from natural resources and structures is an ongoing, significant problem. Because of the serious health problems associated with the bioaccumulation of PCBs in animals, including humans, and the desire for NASA programs to have a quick non-destructive means of removing PCBs from existing structures, numerous tactics have been considered with various degrees of success. Recent research and development work at NASA Kennedy Space Center has led to the development of a reagent comprised of elemental magnesium coated with a small amount of palladium that can be incorporated into a solvent matrix treatment system. Researchers demonstrated rapid and complete dechlorination in aqueous/solvent systems containing chlorinated materials. Additional research has shown that the application of a bimetallic treatment system leads to both the extraction of PCBs from weathered coatings and their ultimate degradation. However, the noble metal palladium coating includes additional processing steps and the palladium material itself provides significant additional costs to the overall treatment system.
In the invention disclosed in U.S. Pat. No. 6,664,298, issued on Dec. 16, 2003, and incorporated into the present application by express reference thereto, a method was disclosed for delivering a reactive material to a contaminant in situ. The method incorporated the concept of either emulsification of the reactant or encapsulation of the reactant prior to its delivery to the contaminant in situ. The method disclosed and claimed in U.S. Pat. No. 6,664,298 has particular success in using a zero-valent metal emulsion containing metal particles, surfactant, oil, and water in a method of enhancing dehalogenation of dense non-aqueous phase liquid (DNAPL) sources. While it is known that zero-valent iron is very effective in the treatment of chlorinated hydrocarbons, such as dissolved trichloroethylene (TCE), zero-valent iron, by itself, is unable to completely dechlorinate PCBs or more robust halogenated compounds such as chlorinated pesticides dissolved in aqueous solutions.
In the invention disclosed in U.S. Pat. No. 7,008,964, issued on Mar. 7, 2006, and incorporated into the present application by express reference thereto, another emulsion system for remediating contaminated media is disclosed. A zero-valent metal emulsion containing zero-valent metal particles doped with a catalytic metal is disclosed to remediate halogenated aromatic compounds, such as PCBs, from natural resources, i.e., in the ground. However, this option for the removal of PCBs found in natural media using an emulsion has several limitations. This emulsion includes emulsion particles comprised of an aqueous interior with bimetal particles encapsulated in a surfactant stabilized hydrophobic solvent membrane. The use of a water-only solvent interior continuum has several disadvantages. Most importantly, making the aqueous-based emulsion requires the potentially hazardous step of adding pure water to the catalytic metal coated zero-valent metal particle. This step is particularly hazardous because:                1. This step produces significant amounts of hydrogen gas which is flammable.        2. The metal particles are so small and light that they produce a dust cloud of catalyzed particles in air when mixed with water. Because of the large surface area of the catalyzed particles, this dust cloud is a potential explosion hazard.        3. The reaction itself is exothermic producing heat that is inherently dangerous in the presence of hydrogen gas.        4. Catalytic metals, such as palladium, within a bimetal particle when mixed with hydrogen gas have the unique ability to produce atomic hydrogen at the metal surface which is extremely reactive. The addition of atomic hydrogen with any of the hazards previously listed increases the likelihood of unexpected explosions or fire.Along with the significant hazards associated with the production of the previous emulsion, the reaction of the bimetal particle with water itself is a competing reaction that affects the dehalogenation of PCBs. Recent laboratory studies have shown that when excess water is in the presence of the bimetal particle for a significant amount of time (greater than 24 hours) before exposure to the PCBs, the PCBs degradation is hindered. This is due to the water depleting the zero-valent metal particle which supplied necessary electrons to the dehalogenation reaction. While the bimetallic particles have been shown to effectively degrade dissolved phase PCBs, the use of bimetallic particles to treat impregnated PCBs would be minimized by the coating material itself.        